1. Field of the Invention
This invention generally relates to semiconductor devices and methods of fabricating the same, and more particularly, to a nitride semiconductor device having a silicon nitride film on a surface thereof as a protection film and a method of fabricating the same.
2. Description of the Related Art
A miniaturized semiconductor device is demanding a higher voltage and a higher power density. A wide band gap semiconductor is highly expected and a number of studies have been made. The wide band gap semiconductor is, for example, silicon carbide (SiC) and a nitride semiconductor such as gallium nitride (GaN) or the like.
In particular, the nitride semiconductor has been developed as an optical device and has been reduced to spectacular practice as a blue light-emitting diode. The nitride semiconductor represented by gallium nitride has characteristics of wide band gap and direct transition. Additionally, the nitride semiconductor also has features including a large breakdown voltage, a high saturation drift velocity, excellent heat conductivity, and hetero junction characteristics. The nitride semiconductor is developed as an electronics device having a high power and high frequency.
In the development of the nitride semiconductor, a MESFET having a non-doped n-type gallium nitride layer was tried to produce in an initial stage. Nowadays, a high electron mobility transistor (HEMT) is a mainstream device. The HEMT utilizes a two-dimensional electron gas, which is one of the features of III-V compound semiconductors. Most of the development of the nitride semiconductor is related to AlGaN/GaN-HEMT.
Miniaturized electronic devices are required to operate at high frequencies. However, the nitride semiconductor device has some unsolved problems of instability deeply related to the surface such as current collapse and gate leakage. The current collapse is a phenomenon, in which the current output decreases when the device produces a large high-frequency power. The surface of the nitride semiconductor has physical and chemical characteristics quite different from those of silicon or gallium arsenide, which have already been studied and a lot of knowledge has been accumulated. Basic studies and developments of process technology are keys for the surface stabilization.
With respect to the gate leakage current and the current collapse that are primary problems relating to the surface of the nitride semiconductor device, it is often speculated that the gate leakage current is caused by a tunneling process through a trap level existent in the interface. However, the detail is not clearly found out.
With respect to the current collapse, there are three models of generation mechanism. The first model is such that, when a stress is applied to a gate, electrons are trapped into an AlGaN surface trap from the gate electrode, and another gate is generated at an edge of the gate electrode. In the second model, when the crystal growth is completed or while the device is being processed, the nitrogen depletion or dangling-bond state occurs on the AlGaN surface. This causes the surface level having a structural defect. The third model assumes that the current collapse arises from hot electrons trapped at a deep level in the buffer layer or the strain induced by the gate bias. However, the current collapse can be suppressed significantly by the surface passivation with a SiN film, and so it is often considered that the current collapse results from the change in a charging state at the surface defect level. The following paper describes that since the SiN/GaN structure was reported to have a low interface level density, the SiN passivation has been effective for the suppression of the current collapse and the improvement in the high frequency characteristics of AlGaN/GaN-HEMT (Tamotsu Hashizume, “Surface control and passivation of gallium nitride—based electron devices”, OYO BUTURI, The Japan Society of Applied Physics, No. 73, the third issue, pp. 333-338 (March, 2003)(hereinafter referred to as Document 1).
Both of the gate leakage current and the current collapse are primary problems related to the surface of the nitride semiconductor device, and are deeply related to a certain defect level existent on the surface or in the vicinity thereof. Fundamental techniques are required for the surface stabilization.
The nitride semiconductor device is very sensitive to the surface state thereof. Generally, a nitride silicon (SiN) protection layer is formed on the uppermost layer of a nitride semiconductor laminate in order to stabilize the surface of the nitride semiconductor device. The SiN protection layer can restrain the gate leakage current and the current collapse to some extent, which are caused when the gate electrode is biased.
However, it is hard to suppress the gate leakage current and the current collapse to a commercial level at which the high characteristic requirements are certainly satisfied. The high voltage and high power density are increasingly demanded together with further requirements for downsizing so that the nitride semiconductor device may operate at high frequencies. It is certainly necessary to stabilize the surface of the nitride semiconductor device with the SiN protection layer sufficiently.